This is not technically a modding tool, but it can and does come in very handy on occasion. It is a tool made by Ron Hoffman and can be found on his page, here. The following is his description of the tool:

Editor for save games created by The Witcher. You can modify the attributes and abilities of the player character (Geralt). You can also unpack the files in a save. This version of the editor is for use with the original version of The Witcher. Using this version of the editor with the Enhanced Edition of the game can result in inventory errors when loading a save because the editor does not support the expanded inventory system implemented in the Enhanced Edition.